The impact of electron–electron interactions in the post-specimen region of ultrafast electron diffraction and dynamic transmission electron microscopy instruments has been studied. Specifically, space-charge induced distortions of ultrafast electron diffraction patterns from single crystal specimens and their dependence on electron bunch-charge, beam energy, energy spread, focusing conditions and specimen thickness have been investigated using the General Particle Tracer code. We have found that these space-charge interactions lead to significant broadening and displacement of the Bragg spots at currently realizable electron beam illumination conditions. These impacts increase in severity with beam brightness and are reduced with increasing (relativistic) beam energies. The primary mechanism for the distortions has been determined to be space-charge interactions between the scattered beamlets and the main unscattered beam. Overall, these results suggest that creative post-specimen electron optical design, relativistic beam energies and post-processing of diffraction patterns to correct for space-charge distortion should be explored as routes to make good use of any future enhancements to beam brightness in UED and DTEM instruments.